Conventional membrane-electrode assemblies for fuel cells are largely classified into two categories: a catalyst-coated membrane (CCM), and a catalyst-coated substrate (CCS). In the CCM among these membrane-electrode assemblies, a catalyst later is applied directly to an electrolyte membrane, and the interfacial properties between the catalyst layer and the electrolyte membrane are excellent. However, there is a disadvantage in that it is not easy to uniformly form the catalyst layer on the membrane surface, because the membrane is deformed by a solvent contained in a catalyst slurry for forming the catalyst layer when the catalyst slurry is applied to the membrane. The most typical method capable of solving this problem is a decal process. The decal process has received attention, because the interfacial properties between a gas diffusion layer, a catalyst layer and a membrane are excellent and the catalyst layer can be formed by a continuous process, and thus mass production is possible.
With respect to the decal process, Korean Patent Laid-Open Publication No. 2004-0104839 discloses a method for manufacturing a fuel cell electrode, comprising the steps of: mixing catalyst particles, a hydrogen ion conductive polymer and a solvent system capable of dispersing or dissolving the hydrogen ion conductive polymer, thereby preparing a catalyst slurry containing the catalyst particles dispersed uniformly therein; applying the catalyst slurry to a polymer substrate and drying the applied catalyst slurry, thereby forming a catalyst layer on the polymer substrate; and placing the polymer substrate on a diffusion layer in such a manner that the catalyst comes into contact with the diffusion layer, and then applying heat and pressure to the resulting structure to transfer the catalyst layer to the diffusion layer, thereby manufacturing the electrode.
And, Korean Patent Laid-Open Publication No. 2009-0132420 discloses a method in which a hot-pressing process is carried out twice in different directions in order to increase the transfer rate of the catalyst layer, and a method in which a release agent is used. This patent publication describes a transfer rate of 100%, which is higher than those of previous patents. In addition, Korean Patent Laid-Open Publication No. 2010-0038543 discloses the results of many studies associated with the decal process, including the composition of solvents for a catalyst slurry, a method of dispersing a catalyst, and conditions for coating and drying the catalyst.
These studies have been conducted on a Nafion® polymer electrolyte membrane (DuPont) having a relatively low glass transition temperature, and for this reason, include a hot-pressing temperature of about 120° C., which is relatively low. But, Nafion® is expensive, so, more inexpensive hydrocarbon-based membranes are currently being developed. But, hydrocarbon-based polymers for preparing these hydrocarbon-based membranes have a glass transition temperature of 200° C. or higher, and thus the hot-pressing process for transfer requires high temperatures that reduce the actual utility of the decal process. In addition, due to a desulfonation occurring at a temperature of 200° C. or higher, the sulfonyl substituent such as sulfonic acid of the hydrocarbon-based membranes should be changed to highly heat-resistant sodium salt before hot pressing, and the sodium salt should be changed again to hydrogen ions after the manufacture of the membrane-electrode assembly. Further, in the substitution process, the hydrocarbon-based membrane comes into contact with a large amount of an aqueous solution, resulting in the deformation of the shape of the membrane-electrode assembly. This is contrary to the advantage of the decal process that prevents the deformation of the shape of membrane-electrode assemblies.
Thus, an advanced and specific decal process for polymers having high glass transition temperatures has been studied. For example, non-patent document 1 (Krishnan et al., International Journal of Hydrogen Energy 35 (2010) 5647-5655) disclosed a decal process showing a transfer rate of 100% under the conditions of 140° C., 8 MPa and 8 min as a result of sequentially depositing a carbon layer, a catalyst layer and an external Nafion® layer on a Teflon polymer substrate and hot-pressing the resulting structure to a hydrocarbon-based membrane. However, the product obtained that process has a structure in which the Nafion® membrane is interposed between the catalyst layer and the hydrocarbon-based membrane, which is similar to the structure obtained by the conventional decal process for Nafion® polymers. Thus, a more advanced and specific decal process that may be applied to polymers having high glass transition temperatures is required.
Accordingly, the present inventors have found that polymers having high glass transition temperatures can be transferred at low temperatures, when the glass transition temperature of a hydrocarbon-based electrolyte membrane is reduced by applying a hydrophilic solvent to a hydrocarbon-based electrolyte membrane in a hot-pressing process required for the decal process, thereby completing the present invention.